1. Field of the Invention
The present invention relates to a solar cell provided with an improved light reflection layer. More particularly, the present invention relates to a solar cell provided with an improved light reflection layer capable of improving the quantity of light absorbed by the semiconductor active layer, which exhibits an improved photoelectric conversion efficiency and which can be provided at a reduced cost.
2. Related Background Art
In recent years, there has been an increased societal demand for early realization of a power generation system capable of providing clean energy without causing CO.sub.2 buildup as in the case of thermal power generation.
There have been various proposals to meet such societal demands. Of those proposals, solar cells are expected to be a future power generation source since they supply electric power without causing problems such as above mentioned.
There are presently a variety of solar cells which have been put into practical use. These solar cells can be categorized roughly into cells made of a single crystal material (which is commonly called "single crystal solar cell"); solar cells made of a polycrystalline material (which is commonly called "polycrystalline solar cell"); solar cells made of an amorphous material (which is commonly called "amorphous solar cell"); and solar cells made of a compound semiconductor material (which is commonly called "compound solar cell").
In order to make a solar cell which is usable as a daily power source, it is essential that large area solar cells can be industrially mass produced at low cost. This requirement is hardly accomplished by either the single crystal solar cells or the polycrystal solar cells. In view of this, amorphous solar cells and compound solar cells have been spotlighted since they can relatively easily be made in a large area and industrially mass produced at a low cost. Various studies have been made of these amorphous and compound solar cells, especially in terms of their production cost and improvement in their photoelectric conversion efficiency.
In any case, one of the most important factors for these solar cells to be usable as a daily power source is that they stably provide a desirable photoelectric conversion efficiency. For this purpose, there have been proposed a number of semiconductor films as the semiconductor active layer which serves to absorb incident light and generate photocarriers causing a photoelectromotive force. In addition, various techniques have been proposed in order to improve their photoelectric conversion efficiency. As one such technique, it is known to dispose a reflection layer comprised of a film of a metal with a high reflectivity such as Ag, Al, Cu, and Au at the side of the solar cell which is opposite the side through which light is impinged. This technique is aimed at improving the photoelectric conversion efficiency by reflecting light which has passed through the semiconductor active layer without being absorbed back into the semiconductor active layer, whereby the output current is increased. Also, there are known a number of other techniques for improving the photoelectric conversion efficiency of a solar cell. For example, U.S. Pat. No. 4,126,150 and Japanese Laid-open patent application 56(1981)- 152276 describe a method of improving the power output of a solar cell provided with a light reflective substrate by using a light reflective member having a roughened surface provided with irregularities as the light reflective substrate to reflect and scatter light which has passed through the semiconductor active layer, thereby increasing the optical path of long wavelength light with a small absorption coefficient. Similarly, U.S. Pat. No. 4,497,974 describes an optically enhanced solar cell provided with a substrate having a surface textured by using a lithographic technique.
Thomas C. Paulick, Journal of Applied Physics, Vol. 62(7), pp. 3016-3024 (Oct. 1, 1987) reports observations of optically reflective characteristics of amorphous silicon solar cells with a textured interface.
As for the method of forming irregularities at the surface of a substrate to be used in a solar cell, there have been various proposals. For example, U.S. Pat. No. 4,252,865 describes a wet-etching method; Japanese Laid-open patent applications 58(1983)-159383 and 59(1984)-82778 describe a sputter etching method and a sand blasting method; and Japanese Laid-open patent application 59(1984)-14682 describes a method of roughening the surface of an aluminum substrate by means of direct current electrolytic etching or chemical etching.
As for the material to be treated to provided a roughened surface, there are various proposals. For example, Japanese Laid-open patent application 58(1983)-180069 describes the use of an organic insulating layer and a metal reflective layer disposed on said organic insulating layer which can be easily treated to provide a roughened surface; and Japanese Laid-open patent application 59(1984)-213174 describes use of a ceramic substrate.
In addition to those above mentioned, Japanese Laid-open patent publication 55(1980)-108780 describes interposition of a transparent and conductive layer between the back electrode and the thin film solar cell semiconductor substrate in order to obtain a high reflectivity back electrode. Similarly, Japanese Laid-open patent application 56(1981)-69875 describes interposition of a transparent and conductive layer between the amorphous semiconductor layer and the conductive substrate in order to relax the conditions required for the mirror state of the substrate surface.